1. Field of the Invention
This invention pertains generally to electroactive polymers, and more particularly to a new category of electroactive polymers that are strong and can be deformed by electrical actuation. The actuation is bistable: the original or deformed shape of the polymer can be maintained until further actuation with the application of a temperature increase and an electric field. The deformation is reversible and can be repeated for numerous cycles.
2. Description of Related Art
Transducers are devices that are capable of converting energy from one form to another or from one system to another. Measurement or input transducers may use a wide range of physical, chemical, or biological effects to create an input variation in one energy form into corresponding variations in another energy form to provide an output. For example, a shape memory polymer is rigid at ambient temperature and rubbery and elastic above its glass transition temperature. A shape memory polymer is mechanically stretched, or “programmed”, above a transition temperature when the polymer is rubbery. The temporarily deformed shape is retained when the temperature is cooled to below the transition point. In deployment, the deformed polymer is heated to above the glass transition temperature at which the polymer recovers its original shape. The requirement of programming is inconvenient for applications wherein multiple cycles of deformation-recovery are required. Furthermore, although a shape memory polymer may be made to restore more than one memorized shape, it cannot be used to obtain a larger number of or gradually-deformed shapes.
In other applications, it is desirable to convert between electrical and mechanical energy. Common examples of applications requiring translation from electrical to mechanical energy include robotics, pumps, valves, general automation, prosthetic devices, orthopedic devices, surgical tools, automotive controls, microelectromechanical systems, adaptive structures, adaptive surfaces, implanted medical devices, optical switches, and adaptive lenses.
Electroactive Polymers or EAP's are polymers whose shape can be modified with an applied voltage to transduce electrical energy into mechanical motion. Some EAP's are characterized by being able to make significant deformation while maintaining large forces.
There are generally two recognized classes of EAP's: dielectric elastomers and Ionic EAP's. Dielectric elastomers, exhibit reversible large actuation strains when a high electric field is applied across a thin film of the polymer. Actuation is achieved by electrostatic forces that squeeze the polymer between two electrodes. However, many electric actuator technologies such as electromagnetic motors and solenoids are not suitable for applications where the required device size is small (e.g., micro or mesoscale machines).
The dielectric elastomers are gel-like soft polymers. In order to generate a large Maxwell strain, a low elastic modulus is required, often less than 10 MPa. As such, the dielectric elastomers lack sufficient mechanical stiffness to undertake a large external load. The actuation of a dielectric elastomer is not bistable. Accordingly, when the driving voltage is removed the polymer returns to its original shape. The lack of bistability can reduce the operation lifetime, particularly for situations wherein the polymers need to maintain the deformed shape for a relatively long period of time.
There are other EAP's such as polyvinylidene difluoride (PVDF) and various copolymers of PVDF that are fairly rigid. Their actuated strain, however, is small, generally less than 10%, and the actuation they are not bistable.
Generally EAP's lack the stiffness required for the structural functions that most synthetic polymers are known for. In addition, the actuation of dielectric elastomers is not bistable: the high electric field has to be maintained in order to preserve the actuated deformation. The lack of bistability makes the dielectric elastomers less suitable for long duty cycle or open loop applications wherein the actuated shape needs to be maintained for an extended period of time. In such occasions, the current leakage at high strains can severely reduce the operation lifetime of the dielectric elastomer actuators.
Accordingly, there is a need for materials and devices that can convert between electrical and mechanical energy with large and bistable actuation. The present invention recognizes and satisfies a need for new electroactive polymers that convert between electrical and mechanical energy that has bistable actuation and is generally an improvement over the art.